pennyroyal, name for two similar plants of the family Labiatae (mint family), usually distinguished as true, or European, pennyroyal ( Mentha pulegium ) and American, or mock, pennyroyal ( Hedeoma pulegioides ). Both have small bluish or purplish flowers in the leaf axils. The European pennyroyal is perennial and prostrate; the American is annual with numerous erect branches. Both contain a pungent oil said to be repellent to insects, especially fleas. The oil, obtained chiefly from the American pennyroyal, is used medicinally. Pennyroyal tea was an old domestic remedy. Other species of Hedeoma are sometimes called pennyroyal, but the false and bastard pennyroyals are different plants. Pennyroyals are classified in the division Magnoliophyta, class Magnoliopsida, order Lamiales, family Labiatae.